Air brake question

notjustair · Oct 2, 2016

1987 Ford L8000 semi.

Last year during harvest I noticed that the air brakes were bleeding off different. It was shorter bursts of air. Like, running down the road it would bleed off six times a mile. Yesterday I ran a load of bales - all still the same. Good pressure and all. Electric brakes on the bale trailer.

Today it is REALLY short cycling. Tiny burst of air bleed off every 15 seconds or so at idle. Air pressure still good. It doesn't matter whether the trailer has air or electric brakes - still the same.

What do I need to do to the bleed off valve?

JD Seller · Oct 2, 2016

IS it the re-dryer dumping off air??? Is the air pressure at the high side kick out pressure??? IF it is and the air is coming from the re-dryer then the air governor is bad. That is the small control valve assembly on the back of the air compressor. They are held on with two bolts. Pretty easy to change.

If it is the pressure dump valve on the air tanks it still is the air compressor governor not shutting the compressor off and it is over pressuring the system.

Here is a picture of the most common air governor on your era of truck. They can be mounted in several ways so that is why there are several plugs included with the new ones. They usually cost around $25-30. The black cover screws off. Under it is a screw with a lock nut. This is how you set the kick off pressure. IT is backwards from a normal pressure valve. Meaning screwing the screw out raises the pressure. In is lower and out is higher. Most trucks kick out at 115-120 PSI

old · Oct 2, 2016

Do you only have the aromatic bleeder or do you have a hand one also?? You could have a lot of water in the system which will cause the automatic one to cycle a whole lot. Been years since I have messed with air brakes much but done some crazy stuff when one ot the trucks would have a brake problem. Still carry a class A CDL

jon f mn · Oct 2, 2016

Sounds to me like your air dryer is dieing.

notjustair · Oct 2, 2016

It's not over pressuring. The tanks aren't bleeding off. It is the unit right behind the driver's side step mounted to the frame rail - I assume it's the drier. It has a blue bottom on it if I recall.

Is that drier a seal thing on the bottom or just replace the drier?

TGP · Oct 2, 2016

Sounds like your looking at the dryer.

You need the make and numbers off it to get a kit, however I would probably just replace the whole thing with new.

It's old enough to retire.
Tom

T in NE · Oct 2, 2016

Drain the tanks, if they have a lot of oil, chances are the governor is plugged up with oil, and the new one will be soon as well.

The air dryer should be rebuildable

john in la · Oct 2, 2016

Is this the piece where the air is bleeding off?
This is the air dryer.
How it works.....

The air passes threw this dryer and stops water and oil from getting into the air tanks.
When the governor shuts off the compressor the compressor unloads.
There is a line that runs from this unload port on the governor to the dryer purge valve.
This air opens the purge valve to allow a burst of air to backwash the dryer.
There is also a check valve in the dryer to protect it.

The check valve or purge valve could be bad in the dryer causing it to purge prematurely.

The governor could also be bad....
Sending air to the purge valve when it should not be
Or its short cycling the compressor with just a small amount of air loss

RodinNS · Oct 2, 2016

It's a while since I worked on one so I don't remember all the tricks... I think from your description you probably have the octogonal Bendix AD9 dryer. Those things have a purge valve that can go bad and also when the dessicant cartridge is failing they will blow off like that. However, as JDSeller has said, the governor can also go bad and cause continuous cycling. I think your problem will be the governor... but if you have no idea the last time the dryer was kitted you might just as well do that too. It seems to me that it can be kitted for 70-100 and perhaps 400 for a replacement (with core). If the can is starting to look suspect I'd just swap the whole thing out and hope they don't hit you for a core charge on the bad can.... If you go the reman route then the purge valve is taken care of too..

Rod

matthies · Oct 2, 2016

I'm with JD Seller the governor is bad, the governor tells the dryer when to purge/dump air. And its the easiest to replace. Also I have seen the air line from the governor to the dryer with a hole and cause the same problem, but you will need someone in the cab to run the engine and shut it off while you are laying under the truck to hear for an air leak from that line cause it doesn't last long after shutdown.

Lazy WP · Oct 2, 2016

If you have an air drier, change the desiccant container.

D beatty · Oct 3, 2016

Does your bleed off valve look like this one? We had ones that looked like this one and company just replaced then and latter in years they took them off and replaced them with a pet cock that we drained daily.All our L series Ford semi tractors had these valves.

MARK ROBKE · Oct 3, 2016

Another thing that can cause the air dryer to cycle rapidly is the signal line from the governor to the drier.It has to hold pressure after it cycles the dryer,until the tank is low.But when the tank is full-governed pressure-the leaking line quickly is refilled with air,and re-signals the dryer to purge.When I had this problem,I would pull the line and,blow into it with a rubber tip blow gun,to isolate it,or the governor.If the purge valve on the dryer continuously leaks after each purge,the dryer is probably at fault.The governor can be the problem,but I found usually it's not.Mark

Air brake question

notjustair

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JD Seller

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old

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jon f mn

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notjustair

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TGP

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T in NE

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john in la

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RodinNS

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matthies

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Lazy WP

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D beatty

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MARK ROBKE

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